E8400 is 45nm so it runs cooler, more cache and higher OC potentail. It is pretty easy for E8400 to reach 4GB(but no every one is able to do this). On other hand, E6600 is 65nm B2 chip so thermal limit at 61c. So, Yes, E8400 is better as long as you can justify the cost because I am assuming you sell e6600 and get e8400.

Temps are basically a zero issue with the 45nm, as long as u have a decent cooler ur fine,
The problem is voltage, if u go to high it fries it, and it wont work anymore
Thats what happened to mine and im trying to sell it to someone that is willing to rma it, if i increase voltage by .1 it wont post, and it has random BSOD's.